The production of copper foil for electronic applications, e.g., copper-clad laminate for printed circuit boards, involves the use of a well-known electrodeposition process. This process utilizes a large cylindrical drum cathode which rotates, partially immersed in a copper sulfate-sulfuric acid electrolyte. The drum cathode is adjacent to and facing toward a pair of curved anodes, which may be formed of lead, lead-antimony, platinized titanium, iridium or ruthenium oxides. Both the drum and the anodes are connected electrically by heavy buss-bars to a D.C. power source and currents of up to 50,000 amps or more are commonly used. As the drum rotates in the electrolyte, an electrodeposit of copper forms on the drum surface, and as the latter leaves the electrolyte, the electrodeposited copper is continuously stripped from the rotating drum in the form of this foil, which is slit to size and wrapped around a take-up roll. The top surface of the drum is usually formed of stainless steel, titanium or chromium.
Foil produced in such a process, prior to being treated, is usually referred to as raw foil. The raw foil is pale pink in color and has two distinctly different looking sides - a "shiny side", the side which was plated onto the drum surface and then stripped is quite smooth while the other side, the side which was facing toward the electrolyte and the anodes, is referred to as the "matte" side since it has a velvety finish. The matte side can be imagined as a set of closely packed cones having heights of from five to ten microns, the cone heights depending upon the independent variables of foil thickness, current density, solution composition, and the like. This provides the basic shape of the foil surface for embedding in the resin of a substrate to promote adhesion in the copper clad laminates used in the manufacture of printed circuit boards (PCB's).
While the matte side of the foil has a certain micro-roughness, a surface bonding treatment is typically applied to the matte side of the raw foil to ensure adequate bonding strength after the copper-clad laminate is formed. The term "bonding treatment" is universally used to refer to changing one or both surfaces of the electroformed foil to make it suitable for bonding to laminate resins.
The bonding treatment operation is conducted in machines called "treaters" wherein rolls of raw foil are unrolled in a continuous manner and fed into the treater by means of driven rollers (similar to the way in which a web of paper is handled in a printing machine), rendered cathodic by means of contact rollers and passes in a serpentine fashion through one or more plating tanks, facing, in each tank, a rectangular anode. Each tank has its own supply of appropriate electrolyte and its D.C. power source. Between the tanks the foil is thoroughly rinsed on both sides. The purpose of this operation is to electrodeposit on at least one side of the foil, usually the matte side, microprojections of complex shape which ensure that the foil will be firmly anchored to the base polymeric materials used in fabricating the copper clad laminates.
High peel strength (the force necessary to pull apart the copper foil and the supporting insulating substrate material) is a characteristic of the highest importance, since the mechanical support of the circuit elements as well as the current carrying capability of PCB's is provided by the copper foil--polymer joint. It is essential that the foil is bonded very tightly and securely to the substrate and also that such an adhesive joint can withstand all the manufacturing steps in PCB's fabrication without the decrease of the initial adhesion, which, moreover should remain constant throughout the service life of the PCB.
This bonding operation is carried in laminating plants and involves heating and cooling cycles. Sheets of copper foil are laid upon sheets of "prepreg" (e.g., glass fabric impregnated with epoxy resin). Both materials are placed in a hydraulic press having heated pressing plates, and the two materials are pressed together under high pressure. At elevated temperatures the resin liquefies and is forced, by the pressure, to flow into the micro-irregularities of the foil surface. This is followed with a second cycle, when both materials are cooled, while the pressure is being maintained, the resin solidifies in the irregularities of the foil surface, and both materials are firmly bonded together and very difficult to pull apart. It is the responsibility of the matte side of the foil to ensure high peel strength.
The matte side of the finished foil, i.e., the base foil plus treatment, refers to the combined effect of the micro-topography of the matte surface of the base foil (electrodeposited at the drum machine) and the bonding treatment plated upon that surface at the treater machine. Both are equally important.
Until only a few years ago the main segment of the total output of PCB's manufactured in the United States was represented by single-sided and particularly double-sided boards. Classical copper foil is an ideal material for the manufacture of such boards.
As shown in FIG. 1, metallographic cross-sectioning of copper base foil 10 reveals that the foil's two opposing surfaces are not the same. While the surface formed next to the drum 12, the shiny side of the foil, even when viewed under great magnification, is relatively flat and smooth, the surface formed next to the electrolyte 14, the matte side of the foil, has micro-peaks and valleys. As shown in FIG. 2, the matte side, after application of the bonding treatment, comprise an extremely dense and uniform coating of spherical microprojections 16 which greatly enhance the surface area available for bonding to the polymeric substrates.
The shiny side of the foil, after the lamination, represents the processing side of the copper clad laminate. As such, it serves as a substrate for image patterning and soldering to ensure the necessary electrical connections between components. In the fabrication of multilayer PCB's (MLB's), the shiny side of the foil also serves as a surface to be treated by chemical means (brown-oxide or black oxide treatment) for bonding purposes.
Although many properties of the copper foil are important in the fabrication of rigid single or double-sided PCB's the peel strength is one of the most important among them. It has to be remembered that copper cladding represents the external surface of the laminate, and that thin copper foil lines can be relatively easily lifted off the surface of the insulating base material if the peel strength is not excellent.
This is why copper foil manufacturers take advantage of the "natural" micro-roughness of the matte surface of the base foil, which at that stage already has a potential "bondability" to polymers, and further enhance it with bonding treatment to achieve the highest possible final peel strength. This is not necessary, or indeed a desirable characteristic of copper foil if it is destined for the manufacture of multilayer boards, which now dominate the PCB market. In the case of MLB's inner layers, copper foil is encapsulated or "sandwiched" between the layers of "prepreg", and moreover, the double-sided laminates for inner layers are quite thin. That raises the need for "low-profile", "not too high peel strength" copper foil so that the laminate's dielectric properties are not adversely affected which frequently is a result of excessive bonding treatment.
On the other hand, the fact that the top-side (shiny side of the foil) is laminated against prepreg that separates it from the next inner layer raises the question of reliability of such adhesive joint. The shiny side of the foil is quite smooth and offers little "bondability." This is why manufacturers of MLB's apply to the top-side of copper tracks a so-called oxide treatment, to enhance their bondability.
It is widely accepted practice in the manufacturing of MLB's to use oxidation techniques to promote the adhesion between copper surface of the inner layers and the prepreg. Without oxide treatment the bond between copper and the prepreg layer is insufficient to withstand thermal shock of reflow soldering.
During the formative years of the multilayer board industry, with relatively less dense patterns of the inner layer circuitry the bond between the prepreg and the base laminate of the inner layers was not considered important. Copper tracks, it was believed, could be encapsulated in the cured prepreg. On the other hand, today's internal circuitry is very dense and most of the bonding is to copper rather than to base laminate. Thus, today the surfaces of copper tracks have to be "adhesion-prone."
The oxide treatment techniques used in the fabrication of MLB's are troublesome, expensive, and create their own technical problems. One, the so-called "pink ring" is a result in the chemical attack on copper oxide layers by the chemicals used in through-hole plating. It is customary now to engage in additional steps of brown-oxide treatment, which is a reduction of cupric oxide treatment to the metallic copper, since the bonding treatment composed of copper is immune to pink ring, as opposed to CuO which is easily dissolved in mineral acids. This reduction step further complicates brown oxide processes and renders them even more expensive.
It has been proposed that a special copper foil provided with the bonding treatment on the shiny side of the foil is better suited to fabrication of MLB's. If the bonding treatment is plated onto the drum side of the foil this results in a lower peel strength e.g., perhaps about 8 lbs./inch than when the same treatment is plated onto the matte side of the foil e.g., about 12 lbs./inch. Nevertheless, such peel strength is more than adequate in MLB's.
With respect to copper foil destined for use in producing MLB's, we have found that the brown oxide "treatment" which is presently applied to the shiny side of the foil and provides a quite low peel strength can advantageously be applied to the matte side of base foil, which by itself, due to its peaks and valleys topography and the resulting micro-roughness, has a considerable peel strength of about 4 lbs/inch, as opposed to the shiny side of the foil, which has substantially no peel strength at all. When this is done, very little brown oxide has to be applied to the matte side of the foil to bring the peel strength to the desired level of, e.g., 7 lbs/inch or so. This reduced amount of brown oxide is much less fragile in terms of structure, than the higher amount of brown oxide that has to be applied to the shiny side of the foil, to achieve the same peel strength. The need for reduction of cupric oxide to metallic copper can thus be eliminated, and the entire process becomes simpler and less expensive, while the quality of MLB's (particularly the dielectric properties and the resistance to delamination due to the solder shock) are improved.
However, the change in the process of manufacturing this special copper foil, when compared to the classical process, requires more than the mere application of bonding treatment to the shiny side, rather than to the matte side, of the base foil.
Since the matte side of this special foil will first be subjected to "imaging" when a circuitry pattern is transferred to a panel, and then to brown oxide treatment, the usual method of "stainproofing"0 the matte side to protect it from tarnishing and oxidation should be reformulated to render it more suitable for use in commercial operations.
What brown oxide treatment for MLB's and micro-etching techniques have in common is that either a sodium chlorite or a sulfuric acid peroxide micro-etching solution has to get to the surface of the copper to produce uniformly the desired reaction or effect. Stainproof layers, therefore, have to be either easily removed by precleaning solutions or be easily penetrable by brown oxide or micro-etch liquids. Excessively tenacious stainproof layers can form an impenetrable shield between a surface of copper and the processing chemicals, delaying the desired reactions or producing obvious non-uniformity.
With the advent of miniaturized electronics, very densely packed printed circuit boards are needed. Miniaturization often requires that the copper foil conductor, or track lines, of today's printed circuit board be as narrow as 5 mils or less. The degree of high definition of fine line circuitry depends on the quality of copper foil manufactured for the electronic industry, particularly on the surface quality of both sides of the foil.
It is the practice in manufacturing printed circuit boards from copper-clad laminate to form the image of the desired printed circuit pattern on the copper surface of the laminate by a photographic technique which leaves the desired pattern formed of a photoresist material on the surface of the copper. For the photographic imaging to be sharp and precise, photoresist has to spread well on the foil's surface and adhere well to this surface.
It is a practice in manufacturing printed circuit board to roughen the surface of the shiny side of the copper foil to achieve good resist adhesion. This roughening also removes tenacious stainproof films which foil manufacturers apply to the foil to protect it from oxidation and staining before it reaches the user. Photoresist does not adhere to the stainproof films, which therefore have to be removed. Thus, roughening of the foil surface serves the purpose of removal of stainproof film as well as changing the copper surface topography from smooth to micro-rough, to facilitate photoresist adhesion which is a condition of good definition of track lines.
This roughening is performed by either mechanical means, e.g., abrasion by brushes, scrubbing with pumice, or chemical means (so-called micro-etching), which is accomplished by subjecting the copper surface of copper-clad laminates to the etching action of oxidizing mineral acids. Such acids attack the smooth surface of the foil along the copper grain boundaries, thus creating pits and pores and changing the copper surface from smooth to micro-rough.
In the fabrication of MLB's copper foil is laminated (bonded) to polymeric substrates twice. First thin, double-sided copper-clad laminates are produced. These laminates are then subjected to image patterning and etching away of unwanted copper to produce the desired patterns of circuitry. Several layers of double-sided boards prepared in such a manner are stacked together, with sheets of prepreg inserted in between to separate dielectrically each inner board from the other. Such a stack of circuit boards and prepreg is then laminated together to form a monolithic multilayer board. Later, holes are punched or drilled through the board in prearranged places and so-called through-hole plating of copper is used to ensure the electrical interconnection between all layers of copper-track conductor lines.
Good bonding is required between the top surfaces of track lines (the surface which was used for image patterning) and the sheets of prepreg, in the course of second (so-called B-stage) lamination.
It is a practice in fabrication of MLB's to subject the inner layer boards, with their patterns of circuitry, to the so-called brown-oxide treatment, which changes micro-topography of the top surfaces of the track lines to improve their bondability to the polymeric prepreg. This brown oxide treatment is produced by immersing the boards in the alkaline solution of sodium chlorite which by its oxidizing action causes the conversation of metallic copper on the top surfaces of exposed copper tracks, into cupric oxide (CuO), possibly in admixture of cuprous oxide (Cu.sub.2 O), depending on the type of the bath and operating conditions. This oxide coating grows in the form of dendritic crystals, perpendicular to the surface of the copper tracks. Thus, the surface area available for bonding to polymeric substrates is increased and improved "bondability" is achieved.
Various patents directed to bonding treatments for copper foil disclose that one or both sides of the foil which is to be bonded to the substrate is subjected to the bonding treatment (U.S. Pat. No. 5,207,889), or that treatment for copper foil that is to be used for lamination to a board comprises electrodepositing a dendritic layer of copper followed by a gilding layer of copper on the side of the foil that is to be laminated to the board (U.S. Pat. No. 4,572,768). Also, the use of either the shiny or matte side of the foil to achieve flexibility in terms of surface characteristics of the resulting copper-clad laminates which have either a mirror-like finish shiny side or "copper clad laminate having a satin finish"(matte side) is disclosed in U.S. Pat. No. 3,998,601. U.S. Pat. No. 3,857,681 discloses the application of copper dendritic and gilding layers to at least one of the surfaces of copper foil to improve the bond strength when laminated to a polymeric substrate, followed by the application of a zinc coating to prevent laminate staining or discoloration.
It is also known to apply a stainproofing chromate layer on the surface of copper foil to protect against tarnish and oxidation, as disclosed in U.S. Pat. Nos. 3,625,844 and 3,853,716.
The matte side of the foil, with its own micro-roughness and the resulting bondability is a better surface upon which to grow the layer of brown oxide, than the traditionally used shiny side of the foil, even if it is roughened by either micro-etching or mechanical abrasion.